1. Technical Field
The present invention relates to an inkjet printer, and more particularly, to an inkjet printer that employs a movable conveyance belt upon which a recording medium is placed and conveyed during printing.
2. Background Art
Movable conveyance belts are used to convey a recording medium such as a sheet of paper during printing. The conveyance belt is used in conjunction with a print head that has a plurality of nozzles defined in its bottom, nozzle face from which ink is expelled in tiny droplets onto the recording medium entering a print zone immediately below the print head. Some print heads have an elongated, stationary configuration extending in a direction perpendicular to that in which the belt conveys the recording medium.
Generally, in stationary-head inkjet printers, the print head is positioned extremely close to the conveyance belt with a uniform gap of several millimeters left between the nozzle face and the recording medium for accurate placement of ink droplets on the recording medium. Displacement and deformation of the recording medium conveyed on the conveyance belt can adversely affect imaging quality. For example, curling causes partial separation the recording medium off the belt surface, making it impossible for ink droplets to land on intended, correct positions on the recording medium.
To stabilize the recording medium in shape and position during conveyance, some inkjet printers are equipped with a charging device that imparts electrostatic charges to the conveyance belt to electrostatically attract the recording medium to the conveyance belt.
For example, an inkjet printer has been proposed that includes a belt charging device that electrifies the conveyance belt to create positive and negative charges alternating each other along the length of belt, as well as a sheet charging device that electrifies at least the leading edge of the recording sheet to a polarity opposite that of a particular portion of the charged belt at which the recording sheet is placed. According to this method, the combined use of the belt and sheet charging devices effectively stabilize the sheet conveyed on the belt, where deposition of opposite charges on the belt and the sheet prevents partial separation of the recording sheet off the belt surface.
One problem encountered when employing electrostatic attraction for stabilizing the recording medium is that charging the conveyance belt results in a strong electric field originating from the charged belt surface and extending over the recording medium placed thereon. If not corrected, presence of such an electric field around the print head would lead to various adverse effects on imaging quality. For example, an electrostatic force exerted toward the nozzle face deflects ink droplets ejected from the nozzles, resulting in misplaced ink dots on the recording medium. In addition, an increased potential at the recording medium causes a back flow of tiny fragments of ink toward the nozzle face, which eventually clogs the nozzles or causes variations in the direction in which ink droplets are directed.
To cope with the problem, another, more sophisticated method has been proposed that employs an electrode for injecting charge of a polarity opposite that of the charged conveyance belt to the recording medium being conveyed on the belt. The charge injection electrode is energized by a power supply controlled according to the surface potential of the recording medium, which can vary depending on environmental or operational conditions, such as a type or thickness of recording medium in use. According to this method, provision of the charge injection electrode reduces the surface potential of the recording medium and neutralizes the electric field created over the recording medium.
Although effective for its intended purpose, the method described above is not sufficient. Injecting opposite charge to the recording sheet to reduce the surface potential of the recording sheet works only to an extent that prevents fragmentary droplets of ink from contaminating the print head, but does not completely remove the electric field created around the print head, causing misplacement of ink dots on the recording sheet. Moreover, power supply control according to the surface potential of the recording medium requires installation of a surface potential sensor, which would complicate the structure and operation of the Inkjet printer.